Known in the art is an internal combustion engine which arranges, in an engine exhaust passage, a three-way catalyst, arranges, downstream of the three-way catalyst in the engine exhaust passage, an NOx storage catalyst which stores NOx in exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean and releases the stored NOx when the air-fuel ratio of the inflowing exhaust gas is made rich, and which switches an engine operating mode in accordance with an engine operating state to either of a lean air-fuel ratio operating mode where the fuel is burned under a lean air-fuel ratio and a stoichiometric air-fuel ratio operating mode where the fuel is burned under a stoichiometric air-fuel ratio (for example, see Patent Literature 1).
In such an internal combustion engine, burning fuel under a lean air-fuel ratio enables a smaller fuel consumption compared with the case of burning fuel under the stoichiometric air-fuel ratio. Therefore, in such an internal combustion engine, usually the fuel is burned under a lean air-fuel ratio in as broad an operating region as possible. However, if burning fuel under a lean air-fuel ratio when the engine load becomes high, the temperature of the NOx storage catalyst rises and as a result the NOx storage ability of the NOx storage catalyst falls, so the NOx purification rate falls. Therefore, in such an internal combustion engine, to prevent the NOx purification rate from falling, when the engine load becomes higher, the operating mode is shifted from the lean air-fuel ratio operating mode to the stoichiometric air-fuel ratio operating mode.